As infrared photodetectors of this type there conventionally are three kinds.
The first type is a two-dimensional quantum well infrared photodetector (QWIP) utilizing a photoconducting phenomenon that accompanies the intersubband excitation of a semiconductor two-dimensional quantum well, which has widely been known as a detector for a mid-infrared range of wavelengths around 3 microns to 20 microns in wavelength (see B. F. Levine, J. Appl. Phys. 74, R1-R81 (1993)). This detector as shown in FIG. 8 in the paper detects free electrons absorbing an infrared light and excited in two-dimensional intersubband transition as an electric current between electrodes across the two-dimensional quantum well. Because of the impracticality to detect a current by one single free electron, however, the detector is far behind the level of detecting single photon and has its sensitivity D* in the order of 1010 cmHz0.5/W.
As the second type, a single photon detector has been proposed and put to realization for a far-infrared region of wavelengths around 100 μm (microns) to 600 μm, which uses a semiconductor quantum dot and a single-electron transistor (see Japanese Patent Laid-Open Application, JP 2001-119041 A; U.S. Pat. No. 6,627,914; S. Komiyama et al., Nature 403, p. 405 (2000); O. Astafiev et al., Appl. Phys. Lett. 80, 4250 (2002)). This detector is so configured that electrons of a semiconductor quantum dot are excited upon absorbing far-infrared photons to leave the semiconductor quantum dot polarized or ionized for a long time period and a change in potential by the polarization or ionization causes the single-electron transistor to be driven. According to this detector in which the semiconductor quantum dot remains polarized or ionized for a long time period, it is possible to detect a single far-infrared photon absorbed since integration of a change in current value of the single-electron transistor gives rise to a value that is detectable in magnitude even for one single far-infrared photon. However, this detector is a measuring device designed to aim at the millimeter wave to far-infrared range and does not operate in the mid-infrared range of wavelengths shorter than several tens of microns. To wit, this detector utilizes a transition between levels whose level distance corresponds to energy in a millimeter wave to far-infrared range such as quantum levels by the in-plane size effect, in-plane plasma vibrations or Landau levels of in-plane orbital motions, and is therefore incapable of detecting infrared lights in a range of wavelengths shorter than several tens of microns. For operation in such a range of short wavelengths as well, a detector must utilize a two-dimensional intersubband transition that is selectively brought about and it requires a new physical mechanism, new both in condensing light and in detection, and a device structure for the same to function.
As the third, there has been proposed a quantum well type detector for detecting single photon by two-dimensional intersubband transition in a semiconductor two-dimensional quantum well (see Japanese patent Laid-Open Application, JP 2004-214383 A). This detector as shown in FIG. 32(a) includes a semiconductor mesa structure 191 and a single-electron transistor 192 on the semiconductor mesa structure 191 wherein the mesa structure 191 has an electron energy barrier which is designed to have a structure as shown in FIG. 32(b). Absorbing an infrared light 190, electrons 194 in a quantum dot 193 in the semiconductor mesa structure 191 bring about an intersubband transition, escaping in a vertical direction and being absorbed in an electrode 195. An ionized state of the quantum dot 193 by escape of electrons 194 continues for a long time period, and the infrared light 190 is detected by detecting a change in the single-electron transistor current by ionized charges 196. Having the structure that a semiconductor quantum dot remains ionized for long time period by two-dimensional intersubband transition, the structure that a single-electron transistor is driven by the ionization potential and the antenna effect by the electrode of the single-electron transistor, this detector is stated to be capable of condensing an infrared light efficiently at the area of a quantum well. In the absence of means for coupling the quantum dot and electromagnetic waves to selectively bring about the two-dimensional intersubband transition, however, the detector is poor in sensitivity. Further, it is difficult to design the quantum well structure such as to set free excited electrons 194 to escape. Moreover, the structure that a single-electron transistor made of a metal or metal oxide such as Al or Al2O3 is fabricated on a quantum well formed of a substrate of a heteroepitaxial semiconductor such as of a GaAs group tends to cause a semiconductor quantum well surface during the fabrication to be oxidized and thereby to deteriorate, and is not suitable for an infrared photodetector in the form of an array which requires quite a high yield.